This invention relates to optical systems for performing measurements, and in particular to an apparatus and method for simultaneous compensation of drift occurring in a light source and in a light detector.
In many fields optical measurement methods are preferred over other approaches because of their non-destructive nature and high accuracy. For example, measurements of reflected or transmitted light can be used to determine numerous physical properties of objects. In fact, optical measurements can be used to study the surface and the interior of objects or of the layers of which the objects are made. The basic physical parameters which can be derived from the study of reflected and transmitted light include thickness, index of refraction, extinction coefficient, surface roughness and energy bandgap of the material making up the object or of a particular layer of the object, e.g., the top layer. Other properties and information about the object and/or its layers, such as material composition, mechanical condition, doping level etc. can be derived from these basic physical parameters.
The prior art teaches optical systems for performing optical measurements using light reflected and/or transmitted by the object being studied. Furthermore, methods for analyzing the reflectance and transmittance data, e.g., spectrum, polarization, intensity and other characteristics of the reflected or transmitted light are also known. For example, in U.S. Pat. No. 4,905,170 Forouhi et al. describe an optical method for determining physical parameters of thin films based on reflectivity or transmittance data obtained over a range of wavelengths.
Further improvements in optical measurement and analysis techniques are frequently hampered by hardware problems. Specifically, light sources delivering the light incident on the samples are subject to intensity fluctuations. Also, light detectors positioned to receive the reflected or transmitted light are subject to fluctuations in detection sensitivity. These fluctuations are typically non-uniform across any given wavelength range and difficult to predict or unpredictable. They are caused by external influences such as temperature, pressure, humidity and operating conditions such as mechanical vibration, current, wear, aging and others.
Over time, source and detector fluctuations add up to produce source and detector drift. In general, the drift in the light source is not related to the drift in the light detector. Thus, in some cases the relative drift between source and detector can be the sum of the drifts.
FIG. 1 shows a simplified prior art optical measurement system 10 having a light source 16 for illuminating a test sample 12 with a probe beam 18, and a detector 20 for receiving a reflected beam 22. Source 16 spans a certain wavelength range. The intensity Is of beam 18 is graphed adjacent source 16 as a function of wavelength xcex at an initial time t0, e.g., at start-up, and at a later time t1, e.g., after several minutes of operation. Likewise, the detection signal Id of detector 20 is graphed adjacent detector 20 at times t0 and t1. At time t1 the source drift and detector drift are clearly significant and not correlated over the wavelength range. Hence, any calibration between source intensity Is and detector signal Id at time to can not be used for compensating the source and detector drifts at time t1.
The prior art teaches to compensate for source drift in optical systems. For example, U.S. Pat. No. 5,054,878 to John S. Gergely et al. teaches a device that automatically compensates for source light drift at the output end. In this invention, the output of the light source is coupled into a fiber optic, and a portion of the light output from the fiber optic is directed into a calibrated photodiode. The calibrated photodiode is connected to a transimpedance amplifier. The remainder of the light output by the fiber optic is focused into a test sample, and the light reflected from the test sample is measured using a detector. The signal detected at the detector is compared with the signal read at the transimpedance amplifier. To compensate for the drift in the light source, the signal from the transimpedance amplifier is divided into the signal from the detector.
The solution of Gergely et al. does not account for detector drift. In fact, most prior art solutions concentrate on stabilizing or compensating the light source by adjusting the external parameters and/or the operating conditions. For example, source intensity is often compensated by adjusting the power supply (operating current) or varying the operating temperature (active heating and/or cooling). Meanwhile, detector drift is either assumed to be negligible and not taken into account or re-calibrated on an infrequent basis.
Advanced prior art optical measurement techniques, e.g., the Forouhi and Bloomer method, are very sensitive to drift. Hence, independent and uncorrelated drift in source and detector will prevent such techniques from yielding accurate values of physical parameters of test samples.
In addition, in some testing environments the optical measurement can not be interrupted to re-calibrate the source and/or the detector. This may be the case when the test sample is located in a controlled environment, e.g., a vacuum chamber, or when the measurement cycle is long and can not be halted. Under these conditions the optical measurement becomes progressively less accurate.
It would be an advance in the art to provide for simultaneous source and detector drift compensation without requiring that the measurement be stopped and without having to access the test sample.
In view of the above, it is the object of the present invention to provide an apparatus and method for simultaneous source and detector drift compensation in optical systems. In fact, the object of the invention is to compensate for drifts occurring in the source and detector over time with minimal disruption to the optical system and without disturbing the test sample.
It is another object of the invention to provide an apparatus for source and detector drift compensation which can be used at any time during a measurement cycle in an optical measurement system.
Yet another object of the invention is to provide a simple and inexpensive apparatus for drift compensation which is easy to use.
Further objects and advantages of the invention will become apparent upon reviewing the below specification.
The objects and advantages of the invention are attained by a method for simultaneously compensating a source drift of a light source and a detector drift of a light detector. A first beam path is provided for a probe beam generated by the source and traveling from the light source to a test location. A second beam path is provided from the test location to the light detector. The beam paths are arranged to intersect or cross at a beam crossing.
A calibration sample is positioned at the test location and illuminated by the probe beam. In response, the calibration sample sends a known response beam along the second beam path to the light detector. The light source and the light detector are calibrated using this known response beam from the calibration sample. For compensation, a reference sample is placed at the beam crossing. The reference sample is positioned such that in response to illumination by the probe beam it sends a reference beam along the second beam path to the light detector. This reference beam is used to simultaneously compensate the source and detector drift. Conveniently, the calibration sample is a highly reflective sample of well-known reflectivity.
The compensation step can be repeated at any time by inserting the reference sample at the beam crossing after calibration of the light source and detector. The compensation is based on a relation established between the known response beam and reference beam. For example, the relation can be established based on the detector current intensities obtained while receiving the known response beam and the reference beam.
During testing a test sample is placed at the test location and illuminated by the probe beam. In response, the test sample sends a response beam along the second beam path to the light detector. In accordance with the invention, compensation of source and detector drift can be performed while the test sample is in place at the test location by inserting the reference sample at the beam crossing.
It is convenient when the reference sample is selected to have a particular response level to the probe beam. In particular, the intensity of the reference beam generated by the sample is preferably within a certain range of the intensity of the response beam from the test sample. It is also convenient when the probe and response beams are collimated at the beam crossing.
A system for simultaneously compensating source and detector drift in accordance with the invention includes the test location, the first beam path and the second beam path intersecting the first beam path at the beam crossing. Furthermore, the system has the calibration sample producing a known response beam to the probe beam and the reference sample for placing at the beam crossing. A first control unit, which controls the sensitivity of the detector, is used for calibrating the light source and detector using the response beam. A second control unit, which controls the intensity of the light source, is then employed for compensating the source and detector drift using the reference beam. In fact, the first and second control units can be integrated in one control unit.
The light source can be any suitable light source spanning the desired wavelength range and can include incandescent bulbs, lasers and gas discharge tubes or any combination of such sources. For example, in measurements requiring reflectance or transmittance data in various portions of the spectrum, the source can be a broadband source made up of a laser and a discharge tube. Correspondingly, a broadband detector or a photospectrometer is chosen as the light detector.
Any known optical elements such as lenses, mirrors, gratings and other beam guiding elements can be used to guide probe and response beams between the source, test location and detector along their optical paths. When using broadband sources and detectors the use of reflective optics such as mirrors in the beam paths of probe and response beams in preferred to refractive optics. In one embodiment, the first beam path has a first mirror, such as a first toroidal mirror for guiding the probe beam. In fact, the first toroidal mirror collimates the probe beam to produce a collimated probe beam. A second toroidal mirror can be positioned to focus the collimated probe beam, e.g., into a fiber for delivery to the calibration sample or directly into the calibration sample. In another embodiment or in the same embodiment the response beam is collimated by a third toroidal mirror to produce a collimated response beam. The collimated response beam is then focused by a fourth toroidal mirrors on the detector. It is preferable that the beam crossing be between collimated probe and collimated response beams. It is further preferred that a first optical path length from the first toroidal mirror to the second toroidal mirror and a second path length from the first toroidal mirror to the fourth toroidal mirror be equal.